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Researchers found for four fifths of the time athletes were exposed to levels of coarse particular matter higher than considered safe by the international health watchdog, the World Health Organization.

For the most dangerous types of soot particles – smaller than 2.5 microns which is more easily inhaled the lungs where they lodge and cause more serious respiratory health problems – these exceeded safe limits for the whole duration of the games.

WHO considered the exceeding of the limits were "excessive" while scientists found levels were a third higher than Chinese government experts claimed.

The findings were revealed as scientists from Oregon State University in the United States of American and their Chinese counterparts from Peking University published the conclusions of the research in the professional journal Environmental Science and Technology.

In the first comprehensive study of particulate air pollutants, scientists took samples before, during and after the games and compared readings from previous games.

American associate professor of environmental and molecular toxicology Staci Simonich said: "Considering the massive efforts by China to reduce air pollution in and around Beijing during the Olympics, this was the largest scale atmospheric pollution experiment ever conducted.

"Despite all that, it was some evening rains and favourable shifts in the winds that provided the most relief from the pollution.

"This demonstrates how difficult it is to solve environmental problems on a short-term, local basis."

Prof Simonich added that despite some favourable weather and the pollution control efforts the games were still the worse – and were about two to four times higher than that of Los Angeles on an average day.

And the difference between the study's findings and the official Chinese figures reflected a difference in measurement methodology – although the study's methods had been widely accepted by the international science community for many years.

But she stressed there was no scientific evidence athletes or spectators suffered health problems during the games.

However Beijing's 17 million population face a long-term struggle against the toxic atmosphere which is said to cause 1 million premature deaths annually across China.

The city is surrounded by mountains that trap air pollutants while rapid industrialisation has seen fumes from factories, cars and coal-fired power stations increase dramatically.

She said: "The athletes and visitors were only exposed for a very short time. Millions of other people there face this air quality problem their entire lives.

"It was unlike anything I've ever seen – you could look directly at the sun and not have a problem, due to the thickness of the haze."